Various photocatalyst materials developed by the group of the present author are described. Alkali and alkaline earth tantalates have arisen as a new group of photocatalyst materials for splitting of water into H2 and O2 under ultraviolet irradiation. They showed activities even without co-catalysts such as Pt, being different from titanate photocatalysts. When NiO co-catalysts were loaded on the tantalate photocatalysts, the photocatalytic activities were markedly increased. Among the tantalates, NiO/NaTaO3 doped with La showed the highest activity. BiVO4, AgNO3, and TiO2 co-doped with Cr and Sb photocatalysts showed high activities for O2 evolution in the presence of a sacrificial reagent (Ag+) under visible light irradiation (λ > 420 nm). Pt/SrTiO3 co-doped with Cr and Sb or Ta, Pt/NaInS2, and Pt/AgInZn7S9 photocatalysts showed high activities for H2 evolution from aqueous solutions containing reducing reagents under visible light irradiation. Furthermore, Cu- or Ni-doped ZnS photocatalysts showed H2 evolution activities even without co-catalysts such as Pt.